The well itself is gone, covered over and replaced by a pump, but the tablet still names it: the Rag Well. The name is the clue to what happened here. People with sore eyes would come to the well, bathe their eyes with a piece of cloth dipped in the water, and then tie that cloth to a bush growing nearby. The rags accumulated, the bush became a kind of votive record, and the name stuck.

The practice is documented in the Schools' Folklore Collection of the 1930s, in which children from Blanchardstown, Castleknock, and St. Brigid's School each recorded what they knew of the well. The Blanchardstown account places it on Darby's Hill, named, according to local memory, for a man called Darby who once owned the land. The ritual as described required visiting the well nine times in total, tying a piece of rag to the bushes on each visit, then drinking the water. The Castleknock account adds further detail: on May Eve, the night before the first of May, people came carrying lighted candles and circled the well in prayer before leaving their cloth offerings. A note recorded by a pupil at St. Brigid's School catches something else entirely; that during a time when all the pumps in Castleknock ran dry, this well alone continued to flow, and neighbours came with buckets to draw from it. The folklore collector and photographer Caoimhín Ó Danachair documented the site in 1958, by which point it had already been covered over.

The site sits at the roadside in Diswellstown, near the Sandpitts area of Castleknock, and the wall tablet is the primary marker of what remains. There is no open water to see, and the bush where rags were once hung is long gone. The tablet itself is modest, easy to pass without noticing. May Eve, the night of 30 April, would once have been the occasion to visit, though the well's active life as a curative site appears to have faded well before living memory.